1. Field of the Invention
The present invention relates to an X-ray CT apparatus that performs cone-beam reconstruction and a method of controlling the X-ray CT apparatus. More particularly, the invention relates to an X-ray CT apparatus that applies X rays to a living subject, providing information about the interior of the subject in the form of an image.
2. Description of the Related Art
In recent years, a system has been defined as a third-generation CT apparatus. This system comprises an X-ray tube and an X-ray detector. The X-ray tube generates an X-ray beam. The X-ray detector is located, opposing the X-ray tube, with the subject lying in between. As the X-ray tube and X-ray detector rotate around the subject, projection data items are acquired from various angles. Hitherto, the X-ray beam is a fan-shaped beam and the detector is a one-dimensional array detector.
Scanning systems are available in two types, i.e., conventional scan system and helical scan system. The conventional scan system is defined as a circular-orbit scan system in which the X-ray tube moves in one circular orbit. The helical scan system is defined as scan system in which the X-ray source and the X-ray detector keeps rotating around the subject and the bed supporting the subject moves along the body axis of the subject. The helical scan system is so named, because the X-ray tube is considered to move in a helical orbit in view of the coordinates system that moves as the subject is moved. Note that the distance the subject is moved along the body axis, i.e., Z axis, every time the X-ray tube rotates around the subject is defined as helical pitch.
Among the third- and fourth-generation CT apparatuses recently developed are a CT apparatus having a new-type X-ray tube and a two-dimensional array detector. The new-type X-ray tube generates not a fan-shaped X-ray beam, but a cone-shaped X-ray beam that diverges, like a cone, in the body axis of the subject. This CT apparatus is called a cone-beam CT apparatus. The two-dimensional array detector has a plurality of one-dimensional array detectors, each consisting of detector elements. These detectors, for example, N detectors, are laid one on another in the Z-axis direction. Thus, the detector elements are arranged in rows and columns, forming a matrix.
Known as a method of providing tomograms, using the conventional scan system, is the circular-orbit, cone-beam reconstruction method proposed by Feldkamp et al. (See, for example, Jon. Pat. Apply. KOKAI Publication No. 2002-360562.) However, cone-beam artifacts develop in the circular-orbit, cone-beam reconstruction method. Consequently, this method cannot provide perfect data. The method has been proved to degrade the image in quality.
In order to solve this problem, a new cone-beam reconstruction method has been proposed. This new method is known as a line+cicle reconstruction method (hereinafter called line-circle reconstruction method). In recent years, Katsevich et al. have proposed a line-circle reconstruction method, which is known as the “filter+back projection” system.
To implement the line-circle reconstruction method successfully, the following requirements should be satisfied.
Since line data is regarded as an additional scan, or an excessive exposure, an additional exposure, if any, must be reduced as much as possible. Further, only a part of line data contributes to the reconstruction and the remaining part is, therefore, a wasteful exposure, though the line data should be large cone-angle data.